Protective device for induction motors



May 19, 1931. c. MACMILLAN 1,805,492

PROTECTIVE DEVICE FOR INDUCTION IOTORS Filed Oct. 23. 1928 Inventor":

Cam beH macmillan,

b5 (,LZZA/ HisAtzizornem Patented May 19, 1931 PATENT OFFICE CAMPBELL MACMILLAN, 01Ev SCHENECTADY, NEW YORK, ASSIGNOR TO GENERAL EL TRIO COMPANY, A CORPORATION OF NEW YORK raoracrrvn nzvrcn ron mnnc'rron MOTORS .App1ication filed October 23, 1928. Serial No. 314,483.

My invention relates to protective devices for induction motors.

It has been the practice heretofore to connect the primary wmding of induction motors to a source of alternating current by circuit breakers which are controlled by a protective device connected in circuit with the primary windings of the motor. These protective devices have ordinarily been responsive only to an electrical condition in the primary circuit of the motor and were not dependent on the speed of the rotor so that they did not afiord com lete protection to the motor. This is due to t e fact that when the s eed'of the rotor is reduced the dissipation of heat from the machine is greatly reduced on account 7 of decreased ventilation, and the current in therotor which would be ermissible at normal operating speedwoilld cause overheating of the motor when the rotor is reduced in speed or stalled.

v The object of my invention is to provide a protective device for induction motors which will disconnect the primary winding from the alternating current circuit at a predetermined current in the rotor at normal speed,

and which will disconnect the primary winding from the alternating current circuit at a-lesser current in the rotor circuit as the speed of the rotor decreases. In this way the motor will be amply protected against overheating throughout the entire range of variation of speed of the rotor because the'maximum current which is permitted to flow in the-.' rotor circuit decreases in accordance with frequency and current controlled thermal rethe decrease in speed thereof and the consequent reduction in its capacity for the dissipation of heat due to decreased ventilation. I accomplish this object by connecting the primary winding of the motor to a source of alternating current and providing an arrangement responsive to a predetermined freguency and current in the rotor circuit for isconnecting theprimary winding of the motor from thealternating current circuit.

In the drawing the single figure diagrammatically illustrates my invention as applied to an induction motor aving a wound rotor and resistance connected thereto for adjusting the torque characteristic thereof.

The terminals 10 of the primary winding of the inductionmotor 11 are connected to alternating current mains 12 by means of a circuit breaker 13 having a trip coil 14 for disconnecting the primary windmgfrom the alternating current mains. The rotor of the induction motor is provided with a windin which is connected to resistances 15 throng slip rings 16 carried by the rotor shaft for controlling the torque-speed characteristics of the motor. I

When the motor is operating at normal speed the air circulation therethrough will be a maximum and the capacity of the machine for dissipating heat is a maximum. If

the speed of the motor decreases the circulaheat will be correspondingly decreased. It

is therefore apparent that as the speed of the motor is decreased the maximum amount of current which is permissible in a rotor circuit without excessive heating is correspondingly decreased.

In accordance with my invention, therefore, I provide an arrangement controlled by the current and speed of the rotor which disconnects the machine from the primar circuit, and is so arranged that the maximum current which is permitted to flow in the rotor circuit before the motor is disconnected from the alternating current circuit is decreased in accordance with the decrease in speed of the rotor. Although any suitable device may be employed for this purpose, I prefer to use a lay- 17 which provides a slight time delay between the occurrence in the rotor circuit of the combined frequency and current required to disconnect the primary winding from the alternating current circuit and the opening of the circuit between them. The thermal relay which I have shown in the drawing for controlling circuit breaker 13, in this manner, includes a-bi-metallic element 18 having a heating coil 19 and contacts 20 which are arranged to complete a circuit between the trip coil 13 and the alternating current mains 12 when the heating coil 19 is energized sufliciently to move the bi-metallic element 18 speed 0 2,1 memes into en ement with the contacts. At full the motor the frequency in the rotor circuit will be quite low and as the speed thereof is decreased due to changes in the adjustment of the resistance or on account of a change in load thereon, the frequency in the rotor circuit will increase. In order to control the heating. of the coil 19 in accordance with the frequency and current in the rotor circuit, I connect the same in the rotor circuit in parallel with the reactance 21, the

' impedance of which increases with'the frequency in the rotor circuit. This impedance is of such value that when the motor is running at normal s eed and the frequency is low in the rotor c rcuit a large proportion of current in the rotor circuit flows through the reactance 21 and the heating of the coil 19 will not be sufiicient to cause the bi-metallic' element 18 to bridge the contacts 20 until the motor is substantially overloaded. As the speed of the motor decreases, however, the

frequency in the rotor circuit increases and l a large proportion of the current flowing in the rotor circuit flows through the heatin coil 19, so that the bimetallic element 18 wi be caused to bridge contacts 20 at very much less current in the rotor circuit than at normal speed of the rotor. In this way the current flowing in the rotor circuit at normal speed does not open the circuit breakers 13 and disconnect the primary winding from the alternating current circuit until it is substantally overloaded, and when the speed thereof decreases the current flowing in the rotor circuit which will disconnect the motor from the alternating current circuit is made small enough to prevent overheating of the motor. Moreover, the rotor current required to disconnect the motor from the alternating current circuit is caused to varydirectly as the speed of the rotor and inversely as the frequency of the rotor circuit.

Inasmuch as the current flowing through the resistances 15 may not be equal on account of the slight difference in'adjustment there- 'of, I prefer to arrange a thermal relay 17 in two of the circuits between the resistance 15 and the slip rings 16 so as to insure openhave disclosed, will occur to those skilled in the art so that I do-not desire my invention -to be limited to the articular arrangement set forth except as efined in the appended claims.

What I claim as new and dseire to secure by Letters Patent of the United States, is:

1; An induction motor, having aprimary winding connected to an alternating current rotor winding for disconnecting said prima and responsive to the combined action 0 frequency and current thereof for disconnect ing said primary winding from said altemating current circuit.

3.'"An induction motor having a primary winding connected toan alternating current circuit and a rotor winding, and means for effecting the disconnection of said prima winding from said alternating current circuit in response to a value of current in the rotor winding which varies inversely with the frequency thereof, said means including a time element device jointly controlled by the current and the frequency of the rotor circuit. 4. An induction motor having a primary winding connected to an alternating current circuit and a rotor winding and means in cluding a thermal responsive device connected in a circuit with saidrotor winding controlled by the frequency and current therein for disconnecting said primary winding from said alternating current circuit.

5. An induction motor having a primary winding connected to an alternating current circuit and a rotor winding, and means including a resistance and reactance connected in parallel in a circuit with said rotor winding controlled by the frequency and current therein for disconnecting said primary winding from said alternating current circuit.

6. An induction motor having a primary winding connected to an alternating current circuit and a rotor winding, and thermal rep sponsive means including a heatin coil and a reactance connected in aralle in said rotor winding controlled byt efrequenc and current therein for disconnecting sai primary winding from said alternating current circuit.

7. An induction motor having a primary winding connected to an alternating current circuit and a rotor winding, a resistor connected in circuit with said rotor winding for controlling the torque characteristic of said motor, and means connected in a circuit with said resistance to be responsive to different values of rotor current as the frequency thereof varies for disconnecting said primary winding from said alternating current circuit.

8. An induction motor having a primary winding connected to an alternating current circuit and a'rotor winding, a resistor connected in a circuit with said rotor winding for controlling the torque characteristic of said motor, means including a thermostatic element for disconnecting said primary winding from said alternating current circuit, and means including a heating coil for said thermostatic element and a reactance connected in parallel in a circuit with said rotor winding for heating said thermostatic element so as to disconnect said primar3 winding from said alternating current circuit upon the occurrence of a predetermined frequency and current in said rotor winding. In witness whereof, I have hereunto set my hand this 22nd dafi of October, 1928.

CAMP ELL MACMILLAN. 

